Fluoroelastomers having excellent heat resistance, oil resistance, and chemical resistance have been used widely for sealing materials, containers and hoses. Examples of fluoroelastomers include copolymers comprising units of vinylidene fluoride (VF2) and units of at least one other copolymerizable fluorine-containing monomer such as hexafluoropropylene (HFP), tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), vinyl fluoride (VF), and a fluorovinyl ether such as a perfluoro(alkyl vinyl ether) (PAVE). Specific examples of PAVE include perfluoro(methyl vinyl ether), perfluoro(ethyl vinyl ether) and perfluoro(propyl vinyl ether). Other fluoroelastomers include copolymers comprising units of TFE and units of perfluoro(methyl vinyl ether).
In order to fully develop physical properties such as tensile strength, elongation, and compression set, elastomers must be cured, i.e. vulcanized or crosslinked. In the case of fluoroelastomers, this is generally accomplished by mixing uncured polymer (i.e. fluoroelastomer gum) with a polyfunctional curing agent and heating the resultant mixture, thereby promoting chemical reaction of the curing agent with active sites along the polymer backbone or side chains. Interchain linkages produced as a result of these chemical reactions cause formation of a crosslinked polymer composition having a three-dimensional network structure. Commonly employed curing agents for fluoroelastomers include the combination of an organic peroxide with a multifunctional coagent, or difunctional nucleophilic curatives such as polyhydroxy compounds or diamines. Nucleophilic curatives require an acid acceptor (e.g. a divalent metal oxide) to be activated.
However, cured fluoroelastomer articles containing acid acceptors may exhibit unacceptably high volume swell, e.g. 50-200 vol. %, that can lead to seal failure, when seals are exposed to certain chemicals such as aqueous acids, coolants or biofuels (e.g. biodiesel or bioalcohol) for long periods of time or at elevated temperatures.